Shock hazard of electric motors

Discussion in 'General Electronics Chat' started by blademan8, Jan 19, 2016.

  1. blademan8

    Thread Starter New Member

    Jan 19, 2016
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    Hello,

    I have a question regarding the shock hazard of common electric motors, the type that you find on table saws, grinders etc. I'd really appreciate the advice of any professional electricians if it's possible.

    In the event that the grounding of the motor fails, could it cause the floor( cement) to become live? What if I touched the motor, could the current go through me a make the ground unsafe for others to stand on? Could it travel up the walls as well?

    My shop is located in a basement so some of the walls are cement and some are wood.

    i also use equipment outside on dirt, grass and gravel that is sometimes wet. I didn't know if that would make things worse or not.

    Thank you very much for any assistance or articles that someone may refer me to.
     
  2. blademan8

    Thread Starter New Member

    Jan 19, 2016
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    Hi,

    I should have also asked if wood flooring would make things safer.

    Thanks again!

    blademan8
     
  3. crutschow

    Expert

    Mar 14, 2008
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    All devices used in the shop or especially outside should definitely be connected through a Ground Fault Interrupter Circuit (GFIC).
    That will open the circuit if there is any current from the hot wire to ground (imbalance in current between the hot wire and common), even if the safety ground is missing.
    Those are available in a socket to replace the standard wall socket or a plug in receptacle.
     
  4. profbuxton

    Member

    Feb 21, 2014
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    If the grounding of an appliance(motor,heater, etc) fails(open circuit) then all bets are off. There is no safety should there be a path from the active(L) to appliance metalwork. You cannot guarantee that any thing You are in contact with will insulate you from a shock.
    That is one of the reasons it is now compulsory(in OZ) for any business to conduct regular safety tests on electrical equipment and maintain records of such.
    These tests are best carried out with automated appliance testers which test for earth continuity(with a known current in case of just one strand left), insulation resistance, polarity. These are pass/fail to a standard.
    Also compulsory now is the installation of RCD(Residual current devices, GFIC to you) in all new buildings for power sockets.
    To directly answer your question, if a motor(or appliance) becomes "live" and you touch it you may get a shock(depends on resistance from your to earth, someone touching you may get a shock(depends on their resistance to earth). That's why rescue kits are provided with a large plastic "hook" to drag "live" persons from danger safely.
    Using electrical equipment outside on dirt, grass, wet conditions is specially dangerous under the above circumstances as ground resistance is fairly low and a real danger of electrocution exist under fault conditions. ALWAYS ensure equipment is tested and use a RCD(GFIC).
     
  5. blademan8

    Thread Starter New Member

    Jan 19, 2016
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    Thank you very much for the replies.

    So how far of a diameter around the floor or dirt do you thing the current would go, both if I was touching the motor and if it was just sitting on a Metal or wooden table?
     
  6. Glenn Holland

    Member

    Dec 26, 2014
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    If the motor insulation fails while you're in contact with earth ground, you risk a shock hazard.

    Regarding your question if the current around your feet would present a risk to another person standing on the ground, that's not likely. Unless the ground is wet and highly conductive, the current diffusion pattern (and current density) around your feet would be small enough to prevent another person from getting zapped.

    However, the diffusion of a high current through the earth is the reason that people can get electrocuted by standing next to a tree that's hit by lightning.
     
  7. #12

    Expert

    Nov 30, 2010
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    There is also a danger zone around power sub-stations. When megawatts of energy go rogue, the danger zone can be dozens of feet in radius. The power in a residence is very much smaller than that, ALMOST to the point that it is not a danger to a second person. However, if a second person grabs you to save your life, they might become the preferred path. It is much better to use a tool to knock you away from danger or knock the wires away from you.
     
  8. MaxHeadRoom

    Expert

    Jul 18, 2013
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    In the case of hand tools, they are generally double insulated and in some instances come with a two pin plug, but if used in hazardous environments and outdoors etc, as already mentioned they should be used with GFI protection.
    Max.
     
    #12 likes this.
  9. BReeves

    Member

    Nov 24, 2012
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    I wouldn't over think this. A modern UL rated tool plugged into a properly grounded receptacle is perfectly safe.
     
  10. djsfantasi

    AAC Fanatic!

    Apr 11, 2010
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    But the receptacle could be hot. I was working in an old building in a dirt floor cellar and touched an armored cable and got quite a hit. Couldn't yell for my helper. My hand contracted around the sheathing. Sheer will power freed me. I felt that I dodged a bullet that day.
     
  11. ErnieM

    AAC Fanatic!

    Apr 24, 2011
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    There is actually a common, approved method of creating a ground by burrying a conductor in a concrete slab.

    It is called an Ufer ground, developed back in WWII and still in common use today.

    So... consider the floor to be a conductor.
     
    Sinus23 likes this.
  12. blademan8

    Thread Starter New Member

    Jan 19, 2016
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    Thanks for the replies guys.

    I am particularly interested in the advice given by Glenn Holland and #12 in reference to the motor not be a shock hazard for others. Did you guys base your info on what you learned in electrical school, or off of some books/articles you read?

    Is there a formula to calculate how far the current would travel from a source of ungrounded current through the floor or ground? ive heard that downed power lines are capable of causing electrocution up to thirty feet through the earth.

    I'd really like to understand the principles of grounding and shock hazards better, but don't really know where to start.
     
  13. #12

    Expert

    Nov 30, 2010
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    Glenn and I have a combined experience of nearly 100 years. That's assuming he started later than I did.:D
     
  14. tcmtech

    Well-Known Member

    Nov 4, 2013
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    Personally as someone who works around electric motors of all shapes sizes and forms on a regular basis I can say with a good deal of certainty that in typical situations like you are describing your concerns are largely unfounded to say the least.

    If the machine in question is sitting on any type of flooring that is dry other than a metal one the ability to get a shock from the machine though your body to the floor to the degree of being dangerous is almost impossible.
    Even if you were barefoot on concrete the likelihood of getting a shock or transmitting one to a second person that would be dad enough to injure either person is extremely unlikely.

    If it was me I wouldn't worry about any of this. The likelihood of an actual occurrence resulting in bodily harm is all but impossible. It would be difficult to produce the effect even on purpose. ;)
     
    #12 likes this.
  15. #12

    Expert

    Nov 30, 2010
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    Yebut...it's a nasty surprise when you feel that spark.:D Still, I agree, You're more likely to bust your elbow jerking away from it than getting any real harm from the electricity.

    I'm talking about U.S.A 120 volt electricity. I never got bit by 220 VAC.:cool:
     
  16. MaxHeadRoom

    Expert

    Jul 18, 2013
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    One of the best books is Soares Book on Grounding by Eustace Soares, it is used by NEC/CEC and published by the IAEI, International Assoc. of Electrical Inspectors.
    I picked up a copy off Abebooks clearing house for $1.00!!.
    As I have an Electricians licence and Industrial Electronics Tech. for UK, I got it to aquant myself with N.A. practice.
    Max.
     
  17. blademan8

    Thread Starter New Member

    Jan 19, 2016
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    Hi Max,

    Thank you for the book recommendation.
    Does it explain why the current from a damaged ungrounded motor will not electrify the floor?

    Thanks again,

    blademan 8
     
  18. MaxHeadRoom

    Expert

    Jul 18, 2013
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    A damaged motor will be a hazard, but if conditions exist that do not conform to international grounding standards then the result will be unpredictable.
    Max.
     
  19. GetDeviceInfo

    Senior Member

    Jun 7, 2009
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    You could look up step voltage and touch voltage. There are basic formulas for substrate resistevity.
     
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